1. Field of the Invention
The present invention relates to an apparatus for actuating a process control element using pneumatic and electrical means.
2. Description of the Prior Art
U.S. Pat. Nos. 3,087,468 and 3,313,212 disclosed pneumatic and magnetic-pneumatic control systems for actuating valves and other apparatus. Both patents utilized pneumatic relays and a balance beam assembly to supply air pressure to a piston located in a cylinder. Unbalancing the beam caused air pressure to be supplied to one side of the piston and removed from the other side of the piston so that the piston moved. The piston was coupled to the balance beam by a spring so that as the piston moved, the balance beam received a force in a counterbalancing direction, forming the feedback necessary to allow the piston to balance at a given location. This movement of the piston and a shaft connected to the piston caused the valve to move as requested by a control input.
The initial imbalance or control signal was provided in U.S. Pat. No. 3,087,468 by a pneumatic bellows assembly appropriately coupled to the beam so that expanding or contracting the bellows caused the balance beam to pivot. The beam became balanced when the piston had moved sufficiently so that the force provided by the spring connected to the piston balanced the force provided by the bellows.
In U.S. Pat. No. 3,313,212 the imbalancing force was provided by a magnetic means using a permanent magnet attached to the balance beam and a coil located near the permanent magnet so that a current in the coil caused a magnetic force between the permanent magnet and the coil. This magnetic force caused the beam to pivot, with balance being restored when the piston spring force balanced the magnetic force being applied.
While the systems performed adequately under ordinary operating conditions, when the control source was removed, in U.S. Pat. No. 3,087,468, when the instrument supply air to the bellows was removed or in U.S. Pat. No. 3,313,212, when the coil current was removed, the actuators caused the piston to travel to full stroke in either the open or closed direction, depending upon configuration and pneumatic connections. This was an undesirable situation because this resulted in reduced control of the system, often when control was critical.
U.S. Pat. No. 4,784,039 disclosed a system which improved on the designs of U.S. Pat. Nos. 3,087,468 and 3,313,212 and similar designs. A motor was connected to a drive unit, which in turn was connected to a control spring, which provided the imbalance or control force. Driving the motor in either direction changed the length of the control spring, hence, the control force, causing the positioner setting to change. When the drive signal was removed from the motor, the motor stopped turning and the control force was set at that point. While this did prevent full travel of the actuator if the control signal were removed, the system required bipolar or reversible drive signals to the motor, somewhat complicating controller design and making use of conventional 4-20 mA electrical control signals difficult.